Apart from the war going on in the comments, allow me to add something insightful (I studied human sexuality in college from a famous sex expert ):
1) Perhaps this is an underlying health problem either physically, mentally, or both. Are you able to orgasm on your own? If so, there's nothing physically wrong with you, it has to be a mental-related issue. Perhaps you get nervous or weirder out and just can't get in the mood. There are alot of reasons to not get in the mood. Maybe you don't like/trust the people you are with enough. Just ideas. If you are having a physical problem though, you should see a doctor immediately, it could be something serious.
2) Are you sure you are not a lesbian or have a gender-related issue? I'm not saying you are/nothing wrong with that but one of my past relationships my girlfriend didn't like physical contact because she was a closet lesbian.
3) There is a word for this type of people I can't think of at the moment, but some women literally don't like having objects inside of their vagina, including sex toys and a penis even though they're heterosexual. If this is the issue, you should see a specialist that can help deal with sex-related problems, they can help you find ways to get comfortable with yourself.
4) Contrary to popular belief on the internet, a man's size does not matter, this has been proven time and again since the 1960's with Masters and Johnson's sex trials. Also a women cannot really be "stretched out", even after children sex is still relatively the same for both parties.
5) Maybe you've just had awful sex partners (in which case you can call me if you live in Western Pennsylvania ) I know alot of guys first times they don't last more than 20 seconds sometimes. That's not long enough to get a women off.

Perhaps the issue in you not having an orgasm with a partner, if attempted with more than one, lies with you and not the man. It is an easy thing to get a woman to orgasm, using the right position and adding fingers and lips to the mixture of thrust.

Perhaps you ought to talk with your partner, indulge in a few desires/fantasies.